1. Field of the Invention
The present invention relates generally to antennas, and more particularly to an antenna omnidirectional in a horizontal plane usable for mobile communications equipment, small-size information terminals, and other radio equipment.
2. Description of the Related Art
Monopole antennas and discone antennas are known as antennas that are omnidirectional in a horizontal plane (hereinafter also referred to as “horizontal-plane omnidirectional antennas”) formed of a conductive base plate and a radiating element.
FIG. 1 is a side view of a conventional monopole antenna 100. Referring to FIG. 1, a coaxial connector 120 is attached to a disk conductor 110 from its lower side so that a center conductor 130 of the coaxial connector 120 extends upward, being isolated from the disk conductor 110. The length h of the radiating element of the monopole antenna 100 is required to be approximately a quarter of the wavelength of an electromagnetic wave of the lowest resonance frequency. At this point, the detailed size of the radiating element is determined depending on the impedance characteristics.
FIG. 2 is a side view of a conventional discone antenna 200. The discone antenna 200 is structured by shaping the center conductor 130 of the monopole antenna 100 like a cone. This shape may also be considered as the one formed by shaping one of the conical conductors of a biconical antenna like a disk. The discone antenna 200 has a conical conductor 210, whose diameter is indicated by d in FIG. 2.
An ideal discone antenna is infinite in size, and is not frequency-dependent. However, in a discone antenna having finite size, the upper limit of its operating wavelength is restricted to approximately four times the length h of the radiating element.
A case where the bandwidth is increased and a case where lower frequencies are covered in the horizontal-plane omnidirectional antenna formed of a conductive base plate and a radiating element as described above are shown below.
FIGS. 3A and 3B are a perspective view and a side view, respectively, of a first conventional antenna 300. As shown in FIGS. 3A and 3B, the antenna 300 includes a skirt part 310 and a top load part 320. The skirt part 310 includes a conical base body 311 and a spiral conductive element 312 formed along the exterior surface of the conical base body 311. The top load part 320 includes a flat base body 321 disposed in the vicinity of the apex part of the skirt part 310 and a meandering conductive element 322 formed on the surface of the flat base body 321.
In this antenna 300, the bandwidth is increased because the meandering conductive element 322 formed on the flat base body 321 has a relatively broad belt-like form and because multiple meandering lines make it possible to achieve multiple resonance. Further, the spiral conductive element 312 formed on the skirt part 310 make it possible to achieve electrical length longer than it appears. Accordingly, the antenna 300 can be reduced in size compared with the conventional discone antenna 200 (see Japanese Laid-Open Patent Application No. 9-083238).
FIGS. 4A and 4B are a side view and a plan view, respectively, of a second conventional antenna 400. As shown in FIGS. 4A and 4B, the antenna 400 includes a conductor 410 having an outer shape like a semioval body of revolution and a flat base plate 420. In the antenna 400, the bandwidth is increased and the size is reduced by shaping the radiating element like a semioval body of revolution or a hemisphere (see Japanese Laid-Open Patent Application No. 9-153727).
However, according to the first conventional antenna 300 (FIGS. 3A and 3B), it is necessary to form a meandering or spiral conductor pattern on the base body 321, and the conductor pattern density should be increased with an increase in the bandwidth, thus resulting in a complicated structure.
On the other hand, according to the second conventional antenna 400 using the flat base plate 420 (FIGS. 4A and 4B), a frequency band in which the antenna 400 is usable is subject to the dimensional elements of the radiating element. Accordingly, the antenna 400 should be increased in size in order to make it usable at lower frequencies.